feat(data-cite): add support for data-cite attribute

[marcoscaceres]

This adds a new foundation feature to ReSpec, allowing authors to directly cite another SpecRef spec in inline text. This is currently very simple, on purpose... so we can build:

<!-- normative -->
It's in the <a data-cite="!DOM">DOM spec</a>.

<!-- informative -->
It's in the <a data-cite="HTML">HTML spec</a>.

That automatically adds href attribute pointing to the DOM spec. It also adds the reference to the References section in the right place.

[marcoscaceres]

So, once this goes in, we can start targeting particular "types" - like methods, etc. that will allow seamless cross linking between specs.

[marcoscaceres]

Thinking about next steps... I think I want:

<a data-cite=spec data-lt="xref in other spec" data-cite-type="">thing</a>

We then need a xref service tho.... it's not clear how BikeShed specs are currently producing their xref.json files, but we need to generate the same thing.

Then we need a central database for those with a RESTful API on the front.

[marcoscaceres]

Merging as it's blocking my other stuff... will fix the async() issue on develop branch, as it's unrelated to this. Would still appreciate any feedback.